The present invention generally relates to systems, apparatus and methods for the determining of the presence of a volatile substance in expired breath. In particular, the systems, apparatus and methods are designed to provide measurements of volatile substances, such as alcohol in blood, quickly and accurately under a wide range of environmental temperatures based on concentrations of the volatile substances in breath.
A number of existing methods to determine alcohol concentration in expired air are described in the literature. These include, for example, catalytic semiconductors, fuel cells and infrared spectroscopy. Infrared (IR) spectroscopy makes use of the specific “finger print” that gas-phase alcohol produces when illuminated by infrared light to determine alcohol concentration. The absorption spectrum of any substance is due to resonant molecular vibrations, which are specific to the atomic bonds within a molecule or compound. From this absorption spectrum, the specific substances and their absolute or relative concentrations within a tested gas or sample can be determined. This technique is highly selective between substances.
The performance of breath-based alcohol sensors is often noted with respect to accuracy over a certain range of measurement. For evidentiary instruments, accuracy of ±5% is frequently required, whereas ±20% is considered adequate for screening and similar purposes. Sensors for the consumer market have lower accuracy. Among these and screening instruments, systematic error caused by the poorly controlled condition of the tested breath is common. For IR-based instruments, systematic errors can be minimized by a calibration procedure with sample gases whose temperature and water moisture mimic expired breath and contain known concentrations of ethanol or other volatile substances of interest. The remaining error behaves as stochastic noise from the sensor signal or signals.